Huge cycles render - not possible to save image?
I think found the reason.

After the last tiles of the render finish blender suddenly needs a lot of RAM. More than I expected for the image buffer. Maybe because it uses floats and not bytes?

During my earlier tries I had a lot of chrome and firefox tabs open (browsers take so much RAM, incredible! :)) and only 2GB of the RAM were free.

When I rendered again, this time I closed most other applications before the rendering finished. With 5GB of free RAM it worked! :)